aligner et centrer element css html

1<nav class="navigation">
2          <div>
3            <ul>
4                <li class=item_1>FindThePrecious.com</li>
5                <li class=item_2>Fellows</li>
6                <li class=item_3>Contact us</li>
7            </ul>
8          </div>
9        </nav>
10
11.navigation{
12  width: 100%;
13  height: 1.8em;
14  
15}
16
17li {
18  display: inline;
19  padding:5px;
20  
21}
22////////////////////////////////////////////////////
23 <section>
24 <div class = "portrait">
25          <img class="demo_portrait_1"
26     src="https://via.placeholder.com/150" alt="portrait"> 
27           <img class="demo_portrait_1"
28     src="https://via.placeholder.com/150" alt="portrait_demo"> 
29          <img class="demo_portrait_1"
30     src="https://via.placeholder.com/150" alt="portrait_demo">
31            </div>
32</section>
33                   //////// css///////////
34.portrait{
35text-align: center;
36}
37
38.demo_portrait_1{ 
39  padding:10px;
40  height: 400px;
41  width: 400px;     
42}
43